test: improved readability in sha self tests

Signed-off-by: Valerio Setti <vsetti@baylibre.com>
diff --git a/library/sha256.c b/library/sha256.c
index 4f30e14..6965820 100644
--- a/library/sha256.c
+++ b/library/sha256.c
@@ -781,15 +781,12 @@
     unsigned char sha256sum[32];
     mbedtls_sha256_context ctx;
 
-    sha_test_sum_t* sha_test_sum;
-
-    sha_test_sum =
 #if defined(MBEDTLS_SHA224_C) && defined(MBEDTLS_SHA256_C)
-                ( is224 ) ? sha224_test_sum : sha256_test_sum;
+    sha_test_sum_t* sha_test_sum = ( is224 ) ? sha224_test_sum : sha256_test_sum;
 #elif defined(MBEDTLS_SHA256_C)
-                sha256_test_sum;
+    sha_test_sum_t* sha_test_sum = sha256_test_sum;
 #else
-                sha224_test_sum;
+    sha_test_sum_t* sha_test_sum = sha224_test_sum;
 #endif
 
     buf = mbedtls_calloc( 1024, sizeof(unsigned char) );